Hafnium: the essentials
Most zirconium minerals contain 1 to 3% hafnium. Hafnium is a ductile metal with a brilliant silver lustre. Its properties are influenced considerably by the impurities of zirconium present. Of all the elements, zirconium and hafnium are two of the most difficult to separate. Hafnium is a Group 4 transition element.
Because hafnium has a good absorption cross section for thermal neutrons (almost 600 times that of zirconium), has excellent mechanical properties, and is extremely corrosion resistant, it is used for nuclear reactor control rods.
Hafnium carbide is the most refractory binary composition known, and the nitride is the most refractory metal nitride (m.p. 3310°C).
Hafnium: historical information
Hafnium was thought to be present in various zirconium minerals many years prior to its discovery, in 1923, which was credited to Dirk Coster and George Charles von Hevesey. It was finally identified in zircon (a zirconium ore) from Norway, by means of X-ray spectroscopic analysis. It was named in honour of the city in which the discovery was made. A number of earlier claims seem less likely.
Most zirconium minerals contain 1 to 3% hafnium and it is their chemical similarity which made their separation difficult. It was originally separated from zirconium by repeated and tedious recrystallization of the double ammonium or potassium fluorides.
Hafnium: physical properties
Hafnium: orbital properties
Isolation: hafnium extraction is always associated with its removal from zirconium as it is a contaminant of all zirconium minerals. Solvent extraction methods are used ot spearate the two metals but the process is not easy. These make use of the differential solubilities of the metal thiocyantes (thiocyanate is SCN-) in methyl isobutyl ketone.